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Placenta – Embryo Folding, Embryonic Cavities and Placenta

by John McLachlan, PhD
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    00:00 Now, let’s look at the placenta itself.

    00:04 We saw in the earlier diagrams how the villi are becoming more complex. This is a view in time with a time dimension along the axis, and it shows the increasing complexity of the villi themselves projecting into the syncytiotrophoblast with maternal blood flowing through the syncytiotrophoblast.

    00:26 Eventually, embryonic blood vessels will develop inside the villi. But you can see the embryonic blood and maternal blood never actually come in direct contact with each other. It always has to cross a membrane composed of the trophoblast cells and the syncytiotrophoblast cells in order to exchange nutrients and gasses between the two of them.

    00:50 Again, here’s a picture which we looked at previously, which just shows you what the illustration might look like in real life. The diagram is very helpful from the point of view of labeling, but the photograph gives you an idea of what it would actually look like.

    01:06 Now, what’s the placenta doing? There are five major functions which are being fulfilled by the placenta. Some of which we’ve mentioned already. So, one of its major purposes is the gas exchange. Oxygen is coming from the mother and being taken up by the embryonic and fetal blood. That requires that that embryonic and fetal blood has to have a higher affinity for oxygen than normal adult human blood so that the developing embryo or fetus can extract oxygen from the maternal blood across that membrane that we discussed previously. Equally, waste gasses will diffuse back across there and into the maternal blood and be dealt with by the mother’s respiratory system after that. It’s not just gasses that are involved, but also nutrients and waste products.

    01:58 So nutrients being supplied by the mother to the developing embryo and fetus and waste products being removed from the embryo and fetus into the mother’s blood, and then dealt with by the mother’s own systems subsequently. And of course, if there are to be some adverse environmental factor present, then it may be able to pass the placenta. We know, for instance, that smoking during a course of pregnancy, will significantly decrease the size of the placenta and make it much more likely that things will go wrong during the course of the pregnancy. However, that’s not the only functions fulfilled by the placenta.

    02:38 It’s also a major site of hormonal action right from very early times so that one of the early signals that pregnancy has occurred is the release of human chorionic gonadotropin into the maternal blood. And that’s the basis of many pregnancy tests. That is mediated by the tissues that will become placenta later on.

    02:59 Then finally, the embryo is itself different. It is genetically different from the mother.

    03:06 So, why is it not rejected in the way that the mother would reject an organ transplant unless she was given drugs to suppress her immune system? What is it that protects the embryo and fetus from being rejected as foreign tissue? The answer is that we still have much to learn about this. But there are aspects of it that we know that are mediated by the placenta itself. So it seems to create an immunologically privileged site which allows the developing embryo and fetus not to be rejected. Of course, if we knew how that worked, that might give us clues for dampening down rejection of tissues in organ transplants.

    03:50 Therefore, that’s a major site of research interest at the moment.

    03:56 This is a picture of a placenta seen after birth. The baby is born first and then the placenta is delivered afterward, as I’m sure you know. This is a slightly unusual one and that the umbilical cord is at the edge. But that’s not a variant that would cause any difficulty or harm during the pregnancy. So, what we looked at in this particular lecture? We’ve looked at folding of the embryo. We’ve looked at the development of the extra-embryonic cavities focusing on the amniotic cavity, and the membranes that surround them. And then we closed by looking at the development and the functions carried out by the placenta.

    04:33 Thank you very much.


    About the Lecture

    The lecture Placenta – Embryo Folding, Embryonic Cavities and Placenta by John McLachlan, PhD is from the course Embryology: Early Stages with John McLachlan.


    Author of lecture Placenta – Embryo Folding, Embryonic Cavities and Placenta

     John McLachlan, PhD

    John McLachlan, PhD


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